Manti Te'o is telling friends ... he knew the truth about Lennay Kekua in early December ... but continued the lie for 20 days for the sake of his team.

Sources connected to Te'o say ... it's true what ND Athletic Director Jack Swarbrick said Wednesday night, that Manti realized he had been hoaxed on Dec. 6, when he got a phone call from "Lennay" ... months after he thought she was dead.

But sources tell us ... Manti didn't tell his coaches about the hoax until Dec. 26 ... because he thought it would be a huge distraction while the team prepared to face Alabama in the BCS Championship.

During the 20 days, we're told Manti struggled over how to handle the situation ... but eventually felt compelled to come clean to his coaches.

Manti is currently training for the NFL combine -- and we're told he's getting a lot of support from the guys he's been training with.